Various types of PM sensors (particulate matter detection sensors) for detecting the amount of PM exhausted from an engine or the like have been proposed. For example, a PM sensor disclosed in JP 59-196453A (corresponding to U.S. Pat. No. 4,656,832) includes a pair of opposed electrodes on an insulating substrate. The accumulation of PM changes a resistance between the pair of the electrodes. By using this property, the PM sensor is configured to detect the amount of PM by measuring the resistance between the electrodes. In this case, a detection circuit connected to a sensor element forms a voltage-dividing circuit configured by a resistance between the pair of opposed electrodes and a predetermined shunt resistance. The detection circuit is configured to output a voltage at an intermediate point of the voltage-dividing circuit as a sensor detected signal.
However, in the PM sensor and the detection circuit, a sensor difference, a variation in an elapsed time, a circuit error or the like may be caused, and thereby the PM amount may be incorrectly detected. For example, if a foreign matter such as a metal piece is attached to an insulating substrate of the PM sensor, or if a weak leakage current flows with the impurities in the insulating substrate, a sensor output which originally should not be produced may be caused, or the sensor output may become larger than a value originally assumed.
In view of the foregoing problems, it is an object of the present invention to provide a sensor controller which can effectively reduce a detection error of a particulate matter detection sensor (PM sensor) and can accurately detect the amount of particulate matter.
According to an aspect of the present invention, a sensor controller is adapted to a particulate matter detection sensor. The particulate matter detection sensor includes an attachment portion to which conductive particulate matter contained in gas is attached, and a pair of opposed electrodes spaced from each other at the attachment portion. Furthermore, the particulate matter detection sensor is adapted to output a detection signal corresponding to a resistance between the pair of opposed electrodes. The sensor controller is adapted to calculate an amount of attached particulate matter based on a sensor detection value from the particulate matter detection sensor. The sensor controller includes: a signal output circuit (e.g., voltage-dividing circuit) connected to the particulate matter detection sensor such that the sensor detection value is changeable in a predetermined output range by the signal output circuit; a heater configured to heat the attachment portion so as to burn and remove the particulate matter attached to the attachment portion; learning means for obtaining the sensor detection value in a burning of the particulate matter by using heat of the heater, for calculating a sensor standard value in a state where the resistance between the pair of opposed electrodes is reduced based on the obtained sensor detection value, and for storing the sensor standard value as a learning value; and correcting means for correcting the sensor detection value based on the sensor standard value stored by the learning means. Here, the heater may include a heating unit for burning and removing the particulate mater by using the heat generated from the heater unit, and a heating means for heating the exhaust gas to a burning temperature of the particulate matter, and the like.
Thus, even when the sensor standard value does not become a limit value in the predetermined output range, an error of the particulate matter detection sensor can be determined, and the sensor detection value can be accurately corrected. As a result, a detection error of a particulate matter detection sensor (PM sensor) can be effectively reduced, and thereby the amount of particulate matter can be accurately detected.